The present invention relates to a device for improving the impaired hearing of a human subject, and more particularly, to an implantable hearing aid device.
In an anatomically normal human hearing apparatus, sound waves, which represent acoustical energy, are directed into an ear canal by the outer ear (pinna) and impinge upon a tympanic membrane (eardrum) interposed, at the terminus of the ear canal, between it and the middle ear space. The pressure of the sound waves effect tympanic vibrations in the eardrum, which then become manifested as mechanical energy. The mechanical energy in the form of tympanic vibrations is communicated to the inner ear by a sequence of articulating bones located in the middle ear space, which are generally referred to as the ossicular chain. The ossicular chain must be intact if acoustical energy existing at the eardrum is to be conducted as mechanical energy to the inner ear.
The ossicular chain includes three primary components, the malleus, the incus and the stapes. The malleus includes respective manubrium, neck and head portions. The manubrium of the malleus attaches to the tympanic membrane at a point known as the umbo. The head of the melleus, connected to the manubrium by the neck portion, articulates with one end of the incus, which provides a transmission path for the mechanical energy of induced vibrations from the malleus to the stapes. The stapes includes a capitulum portion connected to a footplate portion by means of a support crus and is disposed in and against a membrane-covered opening to the inner ear referred to as the oval window. The incus articulates the capitulum of the stapes to complete the mechanical transmission path.
Normally, tympanic vibrations are mechanically conducted through the malleus, incus and stapes, to the oval window and therethrough to the inner ear (cochlea). These mechanical vibrations generate fluidic motion (transmitted as hydraulic energy) within the cochlea. Pressures generated in the cochlea by fluidic motion are accommodated by a second membrane-covered opening between the inner and middle ear, referred to as the round window. The cochlea translates the fluidic motion into neural impulses corresponding to sound perception as interpreted by the brain. However, various disorders of the tympanic membrane, ossicular chain and/or inner ear can occur to disrupt or impair normal hearing.
Various passive mechanical ossicular prosthesis and implantation techniques have been developed in connection with reconstructive surgery of the middle ear. See G. J. Jako, "Biomedical Engineering in Ear Surgery", Otolaryngoloqic Clinics of North America, Vol. 5, No. 1, Feb. 1972, and G. J. Jako, et al., "Conservative Tympanoplasty", American Academy of Opthalmology and Otolarynology, Course 319, presented Oct. 1, 1966.
Miniaturized electronic hearing aid devices which compensate for hearing disorders are also, in general, well known. Various of such devices are adapted to be entirely received within the ear canal or partly or completely implanted within the skull of a subject. Examples of such devices are those disclosed in U.S. Pat. Nos. 3,170,046, issued to L. P. Leale on Feb. 16, 1965; 3,712,962 issued to J. M. Epley on Jan. 23, 1973; 3,764,748 issued to J. P. Branch et al. on Oct. 9, 1973; 3,346,704 and 3,557,775 issued on Oct. 10, 1967 and Jan. 26, 1971, respectively to J. L. Mahoney; 3,870,832 issued to J. M. Fredrickson on Mar. 11, 1975; 4,150,262 issued to H. Ono on Apr. 17, 1979; 4,284,856 and 4,357,497 both issued to I. J. Hochmaier et al. on Aug. 18, 1981 and Nov. 2, 1982, respectively. Further description of such devices is found in T. Ohno, "The Implantable Hearing Aid" (Part I) Audecibel, Fall 1984 and Aritomo et al., "Audiological Assessment of Vibratory Hearing" presented at 17th International Congress of Audiology meeting, Santa Barbara, Calif., Aug. 1984. See also, K. Gyo, N. Yanagihara, and H. Araki, "Sound Pickup Utilizing an Implantable Piezoelectric Ceramic Bimorph Element: Application to the Cochlear Implant", American Journal of Ontology, Vol. 5, No. 4, April 1984.
Perhaps the most interesting of the aforementioned United States patents is the '748 patent which concerns implantable hearing aids including those which are configured for disposition principally within the middle ear space. The approach suggested there provides a transducer, which may be a piezoelectric crystal transducer, capable of converting mechanical vibration, within the ossicular chain into an output voltage. That output voltage may be applied to the area of the oval window to electrically stimulate it and may include a diode to rectify the variable voltage output of the transducer into a pulsating DC voltage to stimulate the auditory nerve. In another variant, the patentees suggest the incorporation of a piezoelectric crystal in the area of the oval window which receives the variable voltage signals from the transducer and vibrates to stimulate the auditory nerve. In any of these approaches, however, the proposed system also utilizes what the patentees regard as the natural distortion-free transmission of sound through the ossicular chain wherever possible. They say that, by virtue of leaving the ossicular chain intact, the acoustic energy impinging upon the eardrum passes through the ossicular chain in a distortion-free manner whereby the sound powered hearing aid they describe needs only supply minimal assistance to the hearing process. The description continues in the '748 patent to note that, as an alternative, the stapes may be removed and the hearing aid physically located in its stead where conditions permit. Under those circumstances, where the stapes is removed, the end of the incus is free-standing and the hearing aid is physically associated with it, such as by means of claimable rings or the like. Thus the hearing aid serves as an integral part of the mechanical linkage in the transmission of forces from the eardrum to the oval window in all events, whether or not the integrity or continuity of the ossicular chain remains unimpaired. That being the case, mechanical feedback through the ossicular chain is a likely consequence, diminishing the overall efficacy of the approach suggested there.
Another example of an implantable hearing aid is described in U.S. Pat. No. 3,882,285 issued to J. A. Nunley et al. on May 6, 1975 and commonly assigned with the present invention. In accordance with the Nunley et al. invention, a self-contained miniature hearing device is implanted in the skull just behind the ear (pinna). The device includes a transducer, such as a microphone, a microphone port, an amplifier and a transmitter for providing a mechanical response to the sound received by the microphone. The microphone port is positioned in the ear canal. The transmitter of the preferred exemplary embodiment of Nunley et al. utilizes a piezoelectric crystal connected to the ossicular chain, preferably to the stapes.
An alternative hearing aid design, the cochlear implant, has received a modest amount of attention in the prior art. The cochlear implant is an electronic device that allows profoundly deaf people to "hear" by electrical stimulation of the auditory nerve fibers within the inner ear. A typical system includes an external microphone, signal processor and transmitter, and an implanted receiver and electrode. The microphone transponds normal sound waves, converting this mechanical sound energy into electrical energy representative thereof. The processor amplifies the electrical energy, filters it and sends it to the transmitter, which changes the electrical signals into magnetic signals. Transcutaneous magnetic currents cross the skin and are received by the implanted receiver, a coil for example, and the signal travels to the cochlea via a wire electrode. Current flows between this active electrode and a nearby ground electrode, preferably disposed in the eustacian tube, to stimulate nerve fibers present in the cochlea. The brain interprets this stimulation as sound. See T. Kriewall, "Why Combine Multichannel Processing With A Single Electrode", Hearing Instruments, June 1985; W. House, D. Bode, and K. Berliner, "The Cochlear Implant: Performance of Deaf Patients", Hearing Instruments, September 1981 (both issues of the above cited publications focus entirely on cochlear implants and are incorporated herein and relied upon). See also U.S. Pat. No. 3,764,748, discussed above, for a variation on this theme.
The prior art systems, however, admit of room for improvement in that these known devices tend to be susceptible to interference by extraneous sounds and/or distortion of the sound ultimately perceived by the subject. Other prior art systems utilize air-induced microphones disposed to be responsive to soundwaves for generating an electrical signal from which the stimulus to the inner ear is ultimately derived. Air-induced microphones are disadvantageous in that the microphone is typically either disposed external to the skull or requires a percutaneous element such as a microphone port or connecting wire. Moreover, the frequency response of air-induced microphones tends not to provide sufficient frequency range for realistic fidelity, and such microphones typically do not provide constant frequency sensitivity across their frequency band.
In the aforementioned presentation, "Conservative Tympanoplasty" by Jako et al., it was proposed that a relatively large piezoelectric crystal pickup transducer be built into the place of the tympanic membrane and directly drive a smaller piezoelectric crystal output transducer placed in the oval window. See Jako et al., supra, at pages 53-54. However, the Jako et al. system has reportedly never been reduced to practice, and the practicability of the system was noted as questionable by the authors themselves (See p. 53).